Relative takes final revenge in kind (5)
I believe the answer is:
niece
'relative' is the definition.
(niece is a kind of relative)
'final revenge in kind' is the wordplay.
'final' says to take the final letters.
'in' indicates putting letters inside.
'kind' becomes 'nice' (I've seen this before).
The last letter of 'revenge' is 'e'.
'e' put within 'nice' is 'NIECE'.
'takes' acts as a link.
(Other definitions for niece that I've seen before include "Family girl" , "[PRIEST'S] daughter?" , "Female relation" , "Son's cousin" , "Brother's daughter" .)